The presently most popular motors used in various industries are squirrel-cage induction motors (SQIMs) and brushless permanent-magnet motors (BLPMMs), where SQIM accounts for more than 90% of all industrial motors in use. The modern SQIM and BLPMM have similar stator designs where distributed copper windings around electrical steel laminates are used to create a rotating magnetic field. However, their rotor designs are different. SQIM has a cylindrical rotor shaft with die-casted aluminum or copper squirrel-cage structure on electrical steel laminates. In contrast, BLPMM has permanent magnets either mounted near the surface, forming a cylindrical rotor or buried inside the electrical steel laminates for a salient-pole rotor. While BLPMM typically achieves better efficiency and power density, SQIM has dominated the industry for more than half a century due to its high reliability, decent efficiency and low production cost.
It is estimated that almost one third of the worldwide generated electrical energy is consumed by industrial motors. The electricity bill can account for up to 95% of the lifetime cost of owning and running these motors. Hence the efficiency of a motor is an obvious key parameter for the industry to optimize. When a variable-speed motor with a high operating efficiency is needed in industrial applications, usually an electrical system called variable-frequency drive (VFD) is used to power and control the connected motor. The basic architecture of a modern VFD's power electronics system includes three distinct stages. First the rectifier stage which is built with either passive power diodes, or active insulated gate bipolar transistors (IGBTs) for regenerative and power-factor-correction functions, and optional input filters. Second a DC link stage which is built with a high-voltage capacitor bank, line filtering chokes, a brake chopper and an optional brake resistor. Finally an inverter stage which is built with IGBTs and optional output filters.
Over time, much effort has been taken to improve both the wiring techniques and electrical steel properties in order to improve the overall efficiency of the motor. A lot of efforts have also been taken to improve the performance and reduce the losses of the IGBTs, the high-voltage capacitors and the filters in the VFD in order to further increase the efficiency of the drive. However, the potential for efficiency improvement diminishes since these technologies have been matured. As the requirements on efficiency and performance increase for variable-speed motor applications, it is necessary to start looking beyond the existing.
In order to enhance the understanding of the problems involved in the current motor constructions, below design details of SQIM and BLPMM will be explained. For state of the art SQIM and BLPMM, most of their motor losses are originated at the stator in the forms of stator winding conductor resistive loss and stator electrical steel iron loss. The designs that affect these two major loss sources are described in the following sections.
As for the stator winding resistive loss, because these motors are designed to operate at AC utility voltages with high stator winding inductance, they typically use a multi-turn distributed randomly wound stator winding 1 that is made with round enameled copper wires 2 as the most popular design approach, such as the example stator winding shown in FIGS. 1A to 1D. FIG. 1C shows a schematic cross-sectional view of such a winding 1 in the electrical steel slot openings 3. Except for its high complexity and the associated cost to manufacture and assemble, this multi-turn distributed randomly wound winding 1 is characterized with several disadvantages. Firstly, the electrical steel slot filling factor, which is the ratio of the total cross-sectional area of the copper 4 of the winding to that of the slot opening 3, is low, typically only between 0.4 and 0.65. This is both because space-consuming electrical insulations 5 and 6 are needed both among individual wires and between wires and the slot wall of electrical steel, and because randomly wound round wires leave a lot of space in between them that is then filled with varnish 7. As these insulation materials and varnish typically have low thermal conductivity, the low slot filling factor leads to a higher current density, worse heat dissipation and higher operating temperature to the copper wires which then all translate to a higher winding resistive loss. Secondly, the end-winding portions 8 of the distributed stator winding that extend out of the electrical steel slots at both ends of the stator, as for example shown in FIG. 1D, are large. These end-wiring portions 8 contribute to the total copper use and to the total stator length and resistive loss but not to the motor's effective torque and power. This is because the winding distribution around the stator requires a lot of insulated wires to go across the slots and cross each other many times in order to connect the whole winding. This, in turn, leads to the end-winding portions taking a lot of space and also contributing a lot to the total stator winding resistive loss.
As for the electrical steel iron loss of SQIM and BLPMM, most of the loss is concentrated on the stator rather than on the rotor. The rotor side core iron loss is low because the rotor electrical steel volume is a lot smaller than that of the stator in an typical in-runner motor design, and because during the motor operation, the magnetic flux on the rotor is either slowly rotating at a slip frequency of only few Hertz, in the case of SQIM, or remains largely static, in the case of BLPMM. The stator side iron loss on the other hand is much higher, firstly because of its much bigger core size and higher magnetic flux rotation frequency, secondly because of the low slot filling factor described above. The slot opening size needs to be larger in the design of SQIM and BLPMM in order to reduce the current density and limit the operating temperature of the stator winding, but this approach inevitably results into a reduction of path width 9 between slots for passing of the magnetic flux in the stator electrical steel, which then leads to a reduction of mutual magnetic coupling between the stator and the rotor of the motor and a higher magnetic flux density in between the slots of the stator electrical steel. These effects then translate into both a high stator electrical steel iron loss and a lower stator winding inductance and a low power factor for the motor. Also because of the motor's low winding inductance and low power factor, they translate into a high magnetization current and hence a high RMS current in the stator winding, which then also adds to the total stator winding resistive loss of the motor.
Because of the design issues described above for SQIM and BLPMM, it is obvious that the present brushless motors with their inherent drawbacks are no longer capable of fulfilling an ever increasing efficiency requirement on industrial variable-speed motor applications. Hence a new variable-speed motor design is needed to further improve the efficiency by solving the problems of SQIM and BLPMM motors and to further reduce the stator losses.
U.S. Pat. No. 5,059,876 discloses the use of a rotary transformer in a brushless motor to provide citation to rotor armature. The output of the secondary transformer winding on the rotor is rectified and fed to the rotor armature. The rotary transformer is a convenient way to provide a brushless structure with a DC fed rotor armature, which is an alternative solution to SQIM and BLPMM. However, the motor still suffers from high losses of the stator having field coils. FIG. 25 of U.S. Pat. No. 5,770,936 discloses a similar concept of utilizing rotary transformer and on-rotor rectification to power a motor at a rotor shaft tip on a machine tool. However, no information is given regarding the design of such a motor except for merely calling it as a DC motor, and variable speed operation of the motor at high efficiency is not mentioned at all.
U.S. Pat. Nos. 5,424,625, 5,491,398, 5,686,805, 5,936,374, 6,049,187, 6,321,032, 7,166,984, and 7,375,488 each discloses a brushless repulsion motor, which is an AC motor having a stator winding excitation and a rotor armature which can be selectively shorted by on-rotor electronics means to produce torque, and the sensing and control electronics and its method to achieve torque and speed control of the motor.